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This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev.
Cite this: DOI: 10.1039/c1cs15078b
Graphene-based composites
Xiao Huang,a Xiaoying Qi,a Freddy Boeyab and Hua Zhang*ab
Received 28th March 2011
DOI: 10.1039/c1cs15078b
Graphene has attracted tremendous research interest in recent years, owing to its exceptional properties.
The scaled-up and reliable production of graphene derivatives, such as graphene oxide (GO) and reduced
graphene oxide (rGO), offers a wide range of possibilities to synthesize graphene-based functional
materials for various applications. This critical review presents and discusses the current development of
graphene-based composites. After introduction of the synthesis methods for graphene and its derivatives
as well as their properties, we focus on the description of various methods to synthesize graphene-based
composites, especially those with functional polymers and inorganic nanostructures. Particular emphasis
is placed on strategies for the optimization of composite properties. Lastly, the advantages of graphene-
based composites in applications such as the Li-ion batteries, supercapacitors, fuel cells, photovoltaic
devices, photocatalysis, as well as Raman enhancement are described (279 references).
1. Introduction
The direct observation and characterization of a mechanically
exfoliated graphene monolayer by Novoselov et al. in 20041
has sparked the exponential growth of graphene research in
both the scientific and engineering communities. Graphene, a
single-layer carbon sheet with a hexagonal packed lattice
structure, has shown many unique properties, such as the
quantum hall effect (QHE), high carrier mobility at room
temperature (B10 000 cm2 V1 s1),1 large theoretical specific
surface area (2630 m2 g1),2 good optical transparency
(B97.7%),3 high Youngs modulus (B1 TPa)4 and excellent
thermal conductivity (30005000 W m1 K1).5 To further
exploit these properties in various kinds of applications,
versatile and reliable synthetic routes have been developed
to prepare graphene and its derivatives, ranging from the
bottom-up epitaxial growth69 to the top-down exfoliation
of graphite by means of oxidation, intercalation, and/or
sonication.1013 In particular, the low-cost and mass production
of chemically exfoliated graphene oxide (GO)1429 and reduced
graphene oxide (rGO) sheets has been realized,1423,3034 which
possess many reactive oxygen-containing groups for further
functionalization and tuning properties of GO or rGO sheets.
With these added advantages, it is desirable to harness the
useful properties of graphene and its derivatives in composites,
through the incorporation with various kinds of functional
materials. To date, graphene-based composites have been
successfully made with inorganic nanostructures,1820,3538
organic crystals,3940 polymers,14,17,41,42 metalorganic frame-
works (MOFs),4345 biomaterials,4648 and carbon nanotubes
(CNTs),4953 and are intensively explored in applications such
as batteries,37,5456 supercapacitors,36,5759 fuel cells,6063
photovoltaic devices,19,33,64,65 photocatalysis,6669 sensing
platforms,46,47 Raman enhancement7072 and so on. In this
a School of Materials Science and Engineering, NanyangTechnological University, 50 Nanyang Avenue, Singapore 639798,Singapore. E-mail: [email protected], [email protected];Web: http://www.ntu.edu.sg/home/hzhang/
b Centre for Biomimetic Sensor Science, Nanyang TechnologicalUniversity, 50 Nanyang Drive, Singapore 637553
Hua Zhang
Hua Zhang studied at Nanjing
University (BS, MS), andcompleted his PhD at Peking
University (with Zhongfan Liu)
in 1998. After postdoctoral work
at Katholieke Universiteit
Leuven (with Frans De
Schryver) and Northwestern
University (with Chad Mirkin),
and working at NanoInk Inc.
(US) and the Institute of Bio-
engineering and Nanotechnology
(Singapore), he joined
Nanyang Technological Uni-
versity in July 2006. He was
promoted to a tenured associateprofessor in March 2011. He is an Associate Editor of the
International Journal of Nanoscience, on the Editorial Board of
NANO, and on the Advisory Committee of IOP Asia-Pacific.
His research interests focus on carbon materials (graphene and
CNTs) and their applications; controlled synthesis, characteri-
zation and application of novel nanomaterials; and surface
fabrication from the micro- to nanometre scale, etc. He has
published 25 patent applications, 3 book chapters, more than 130
papers with over 2400 citations and an H-index of 26. He has
been invited to give more than 70 Keynote Lectures and Invited
Talks in international conferences and universities, and serve as
Session Chair. He has organized several international confer-
ences and served as Symposium Chair.
Chem Soc Rev Dynamic Article Links
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Chem. Soc. Rev. This journal is c The Royal Society of Chemistry 2011
critical review, after introducing the properties and synthetic
methods of graphene and its derivatives, graphene-based
composites are discussed, with particular emphasis placed on
their fabrication/synthesis methods, properties and applications.
2. Properties of graphene
2.1 Electronic propertiesOne of the most important properties of graphene is that its
charge carriers behave as massless relativistic particles, or the
Dirac fermions.1 It has been demonstrated that graphene is a
zero-gap 2D semimetal with a tiny overlap between valence
and conductance bands, and charge carriers move with little
scattering under ambient conditions.1 It also exhibits a strong
ambipolar electric field effect with the concentration of charge
carriers up to 1013 cm2 and room-temperature mobilities of
B10000 cm2 V1 s1, when the gate voltage is applied.1
Besides, the suspended graphene shows low-temperature
mobility approaching 200 000 cm2 V1 s1 for carrier densities
below 5 109 cm2, which is not observable in semiconductors
or non-suspended graphene sheets.73 Additionally, an unusual
half-integer quantum Hall effect (QHE) for both electron and
hole carriers in graphene has been observed by adjusting the
chemical potential with the use of the electric field effect.74,75
Such QHE can be observed at room temperature as well,76 and
the fractional QHE was obtained when suspended graphene
devices were probed, which allows for the isolation of the
sample from substrate-induced perturbations.77
2.2 Optical properties
The measured white light absorbance of suspended single-
layer graphene is 2.3% with a negligible reflectance (o 0.1%),
and this absorbance increases linearly with the layer numbers
from 1 to 5.3 The measured values and the observed linearity
are in consistence with the theoretical calculation results
obtained with a model of non-interacting massless Dirac
fermions. It shows that the dynamic conductivity of graphene
in the visible range, G, only depends on the universal
constants, i.e. G E pe2/2h, where c is the speed of light, h is
Plancks constant.78 The transparency of graphene only
depends on the fine-structure constant a = 2pe2/hc, which
describes the coupling between the light and relativistic
electrons. The absorbance of n-layer graphene can thus be
simply expressed as npa. However, deviation from this
behavior is found with the incident photons with energy less
than 0.5 eV (or wavelength larger than B2480 nm), which is
attributed to the effects of finite-temperature, doping, and
intra-band transitions.79
The ultrafast optical pumpprobe spectroscopy has been
applied to study the carrier dynamics and relative relaxation
timescales of graphene layers grown on SiC.80 An initial fast
relaxation transient (70120 fs) followed by a slower relaxa-
tion process (0.41.7 ps) has been identified, which is asso-
ciated to the carriercarrier intra-band and carrierphonon
inter-band scattering process, respectively. With the help of
infrared spectroscopy, it is found that the inter-band transi-
tions and optical transitions of monolayer and double layer
graphene are layer-dependent and can be modulated through
the electrical gating, which holds promise for the infrared
optics and optoelectronics.81
2.3 Thermal properties
The thermal conductivity of single-layer suspended
graphene at room temperature has been measured as
30005000 W m1 K1 depending on the size of the measured
graphene sheet.5 It is supported on amorphous silica, a case
similar to the practical application, the measured thermal
conductivity of graphene is B600 W m1 K1, about one
order lower than that of the suspended graphene sheet.82 This
reduction in thermal conductivity is attributed to the leaking
of phonons across the graphenesilica interface and strong
interface-scattering. Nevertheless, this value is still about
2 times and 50 times higher than copper and silicon, respec-
tively, which are used in the electronics today.
2.4 Mechanical properties
The intrinsic mechanical properties of free-standing mono-
layer graphene membranes were measured based on the
nanoindentation using atomic force microscopy (AFM).
4
The breaking strength is 42 N m1 and the Youngs modulus
is 1.0 TPa, suggesting the strongest material ever measured.
Theoretical work has been carried out to investigate the
mechanical properties of zigzag graphene and armchair graphene
nanoribbons, suggesting that the critical mechanical loads for
failure and buckling of armchair ribbons are smaller than
those for zigzag ribbons.83 In addition, external mechanical
loads can change the electronic properties of graphene, such as
field emission performance.83,84 By employing a cantilever
beam, single-layered graphene can be subjected to various
degrees of axial compression.85 The uptake of stress and the
compression buckling strains of different geometries have thus
been measured. It is found that the mechanical response isrelated to the Raman shift of the G or 2D phonons of graphene.
The elastic deformation of functionalized graphene sheets
(FGSs), or chemically reduced graphene oxide (rGO), has also
been studied by AFM.86 After repeatedly folding and unfolding
of the FGSs multiple times, the folding lines were found to
appear at the same locations, which can be attributed to the
pre-existing kinks or defect lines in the FGSs.
3. Synthetic methods
Till now, tremendous efforts have been made to develop
synthetic methods for graphene and its derivatives, not only
to achieve high yield of production, but also to realize the
solution or thin film based process. These methods can be
generally classified as the bottom-up and top-down approaches.
Table 1 summarizes and compares the typical synthetic
methods for graphene and its derivatives. The bottom-up
approach involves the direct synthesis of graphene materials
from the carbon sources, such as the chemical vapor deposition
(CVD), which is a typical method used to grow large-area,
single and few-layer graphene sheets on metal substrates, e.g.
Ni, Ru, and Cu.69,49 Plasma enhanced CVD (PECVD) is able
to grow single-layer graphene with high throughput at shorter
reaction time and lower deposition temperature compared to
the CVD process.87 Besides, graphitization of carbon-containing
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substrates, such as SiC, can give rise to single-layer and
few-layer graphene films as well through the high temperature
annealing.88,89 In addition to these methods based on the
solid-phase deposition, graphene is also obtainable via the
wet chemical reaction of ethanol and sodium followed by
pyrolysis,90 or through the organic synthesis to give graphene-
like polyaromatic hydrocarbons.91,92
Different from the bottom-up approaches, the top-down
approaches are advantageous in terms of high yield, solution-
based processability, and ease of implementation, which have
been demonstrated by means of intercalation, chemical func-
tionalization, and/or sonication of bulk graphite. The first
observation of exfoliated graphite dates back to 1840 by
Schafhaeutl, when H2SO4 was used for the intercalation.93
Since then, a number of chemical species have been found to
form intercalated compounds with graphite.9497 Further
attempts, combining the intercalation and sonication, have
realized isolation and dispersion of graphene sheets by using
intercalates such as N-methyl-pyrrolidone (NMP)11 and
sodium dodecylbenzene sulfonate (SDBS)13 in non-aqueous
and aqueous solutions, respectively. Re-intercalation of
thermally exfoliated expandable graphite (EG) with oleum
(fuming sulfuric acid with 20% free SO3) and tetrabutyl-
ammonium hydroxide (TBA) has also produced single-layer
graphene sheets with a yield of B90% after purification.98
Even without sonication, ternary potassium salt-intercalated
graphite readily gives isolated graphene sheets in NMP.12
Additionally, electrochemical exfoliation of graphite has also
achieved graphene nanosheets in ionic liquids.99 Unfortu-
nately, the aforementioned methods face drawbacks such
as the low yield of single-layer production,13 expensive
intercalates,11 and residual surfactant induced low conductivity.
Therefore, an alternative approach, the reduction of highly
oxidized graphene oxide (GO) sheets from the exfoliated
Table 1 Comparison of various methods for preparation of graphene and its derivatives
Methods Conditions Yield and properties Ref.
Bottom-up approachesCVD Carbon sources: CH4, H2 Sheet size of up to a few tens of micrometres. 68
Substrate: Ni, Ru, CuTemperature: 1000 1C
PECVD Carbon source: CH4, H2 Large area of more than 1 cm of monolayergraphene
87Substrate: Cu
Temperature: 650 1CGraphitization Substrate: 6H-SiC(0001) Grain size: up to 50 mm long, 1 mm wide 89
Temperature: 1280 1CSolvothermal Reagents: Na and ethanol Folded graphene structures 90
Temperature: 220 1C Bulk conductivity: B0.05 S m1
Organic synthesis Thermal fusion of polycyclic aromatichydrocarbons at 1100 1C
For a 30 nm thick film on quartz, conductivity:20600 S m1, sheet resistance: 1.6 kO m2
91
Top-down approachesLiquid exfoliation ofgraphite
Intercalate: NMP Single-layer yield: 712 wt% after purification 11Film conductivity: B6500 S m1
Intercalate: SDBS Film conductivity: B35 S m1 13Single-layer yield: B3% with a size ofB1 mm
Intercalate: ternary potassium salt Graphene ribbons with lengths ofB40 mm. 12Thermal exfoliation andliquid intercalation
Thermal exfoliation at 1000 1C Single-layer yield: B90% after purification 98Intercalates: oleum and TBA Size: B250 nm
Resistance of single sheet with 100 nm in width:1020 kO
Electrochemicalexfoliation
1-Octyl-3-methyl-imidazolium hexafluoro-phosphate as electrolyte; graphite rods as electrodes
Sheet size: 500 700 nm 99
Chemical reductionof GO
Reduction agent: hydrazine Sheet resistance of graphene paper: 7200 S m1 31Deoxygenation agent: KOH or NaOH Incomplete removal of oxygen-containing groups 109Temperature: 5090 1CReduction agent: bovine serum albumin To be used as template for nanoparticle synthesis 111Reduction agent: vitamin C Film conductivity: up to 7700 S m1 133Temperature: 95 1CReduction via bacteria respiration Film resistance decreased up to 104 after reduction 134Reduction agent: hydriodic acid and acetic acid Sheet resistance: pellets dried after solution
reduction: 30 400 S m132
In solution at room temperature or in vaporat 40 1C
Thin film after vapor reduction: 7850 S m1
Sonolytic reduction: ultrasonication at 211 kHzfor 30 min
Formation of 14 layers of rGO 127
Microwave-assisted reduction in the presence of
hydrazine
Formation of 18 layers of rGO with size up to a
few micrometres
128
Thermal reductionof GO
220 1C in air for 24 h Film sheet resistance: 8 kO sq1 124
150 1C in DMF for 1 h Film resistance: 6 kO 119Photothermal reductionof GO
High pressure Hg lamp with H2 or N2 flow Sheet size: B1 um 115Single-sheet conductivity: 200020000 S m1
Pulsed xenon flash Able to prepare a GO/rGO patterned film with aphotomask
126
Sheet resistance of rGO area: B9.5 kO sq1
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graphite oxide has been developed,10 and mostly employed to
prepare reduced GO (rGO), chemically converted graphene
(CCG), or functionalized graphene sheets (FGS). The graphite
oxide is obtained by the Hummers method via the reaction of
graphite with a mixture of potassium permanganate (KMnO4)
and concentrated sulfuric acid (H2SO4).1423 The exfoliated
GO sheets are thus highly oxidized and featured with the
residual epoxides, hydroxides and carboxylic acid groups on
their surfaces.100,101 So far, various types of reduction methods
have been reported to obtain rGO sheets, such as the chemical
reagent reduction,1422,31,33,102112 photochemical reduction,113116
thermal reduction,117124 photothermal reduction,125,126
sonolysis,127 microwave-assisted reduction128 and electro-
chemical reduction.129132 Among these methods, the chemical
reduction is the most versatile one with many reduction agents
being used, such as hydrazine,1422,31,33,102108 strong alkaline
media,109 vitamin C or ascorbic acid,110,133 bovine serum
albumin (BSA),111 bacterial respiration,134 and hydriodic
acid.32,135 To date, the rGO sheets reduced by hydriodic acid
and acetic acid have shown the best electronic conductivity
(up to 30000 S m1).32 Due to the less toxicity of hydriodic
acid, this method is expected to replace the hydrazine
reduction for the mass production of rGO dispersions and
fabrication of rGO thin films. Despite the fact that the
conductivity of rGO is orders lower compared to the pristine
graphene due to the presence of residual oxygenated groups
and defects, the reactive surfaces of GO and rGO provide the
tunability in electronic and optoelectronic properties via
chemical reactions,118,136,137 and the feasibility for composite
incorporation.138
4. Grapheneinorganic nanostructure composites
In the last few decades, huge efforts have been made to
synthesize inorganic nanostructures with controlled shape,
size, crystallinity and functionality. These materials are widely
employed in applications like electronics, optics, electro-
chemical energy conversion and storage, solar energy harvesting,
and so on. In order to further enhance their properties, a great
number of inorganic nanostructures have been composited
with graphene and its derivatives, which include metals like
Au,20,35,111,114,127,139146 Ag,20,146150 Pd,128,141,146,151,152
Pt,111,141,146,153 Ni,154 Cu,128 Ru155 and Rh;155 oxides like
TiO2,67,68,113,156162 ZnO,19,163165 SnO2,
166,167 MnO2,168,169
Co3O4,54,55,170,171 Fe3O4,
172175 NiO,176 Cu2O,18,102,177
RuO2,59 and SiO2;
178 chalcogenides like CdS179182 and
CdSe.183,184 The fabrication methods are generally classified
as the ex situ hybridization, and in situ crystallization. Table 2
has summarized the typical synthetic methods for graphene
inorganic nanostructure composites and their related applications.
4.1 Ex situ hybridization
The ex situ hybridization involves the mixture of graphene-
based nanosheets and pre-synthesized or commercially
available nanocrystals in solutions. Before mixing, surface
modification of the nanocrystals and/or graphene sheets is
often carried out, so that they can bind through either the
non-covalent interactions or the chemical bonding. For example,
2-mercaptopyridine modified Au nanoparticles (NPs)145 or
benzyl mercaptan-capped CdS NPs179 have been successfully
attached to GO or rGO surfaces via the pp stacking. Alter-
natively, rGO sheets can be modified with adhesive polymers
for anchoring the NPs, which has been demonstrated in the
Nafion-coated rGO/TiO2 composites.161 Amphiphilic bio-
polymers, such as bovine serum albumin (BSA) protein, have
also been used to modify the rGO surface via the pp inter-
action, which then serve as a universal adhesive layer to
absorb Au, Ag, Pt and Pd NPs.146
Instead of decorating NPs on GO/rGO surfaces, GO/rGO
sheets can wrap around the oxide NPs for specific applications
such as the Li-ion battery.55 As reported, after the positively
charged SiO2 or Co3O4 NPs (modified by aminopropyl-
trimethoxysilane, APS) are encapsulated by negatively
charged GO sheets through the electrostatic interaction, the
GO can be in situ reduced to rGO subsequently without
destroying the sheet-encapsulated-particle structures.
4.2 In situ crystallization
Although the ex situ hybridization is able to pre-select nano-
structures with desired functionalities, it sometimes suffersfrom the low density and non-uniform coverage of the nano-
structures on the GO/rGO surfaces.145 In contrast, the in situ
crystallization can give rise to uniform surface coverage of
nanocrystals by controlling the nucleation sites on GO/rGO
via surface functionalization. As a result, the continuous film
of NPs on graphene surfaces can be realized.
4.2.1 Chemical reduction method. Chemical reduction is
the most popular strategy for synthesis of metal nano-
structures. Precursors of noble metals, such as HAuCl4,
AgNO3, K2PtCl4 and H2PdCl6, can be simply in situ reduced
by reduction agents like amines, NaBH4, and ascorbic acid.
For example, grapheneAu NP composites can be obtained by
the reduction of HAuCl4 with NaBH4 in a rGO-octadecyl-
amine (ODA) solution.35 However, since rGO contains much
less residual oxygen-containing functional groups than GO, it
is restricted to aqueous solution based processes. To address
this problem, 3,4,9,10-perylene tetracarboxylic acid (PTCA)-
modified rGO sheets have been used as templates for the in situ
reduction of HAuCl4 with amino-terminated ionic liquid
(IL-NH2). Because of the additional carboxylic groups from
PTCA, the resulting rGOAu NP composites are water soluble
and give a dense coverage of NPs.139
Anisotropic metal nanostructures, such as nanorods143,185
and snowflakes,142 have also been synthesized on rGO or GO
surfaces via chemical reduction. For example, Au nanorods
can be prepared on rGO films or seed-modified rGO films, by
using a shape-directing surfactant, cetyl trimethylammonium
bromide (CTAB), to facilitate the one dimensional growth of
the nanorods. Very recently, by using GO as a synthetic
template, we have synthesized square-like Au nanosheets with
an edge length of 200500 nm and a thickness of B2.4 nm
(Fig. 1A).38 These Au nanosheets display the unique hexa-
gonal close packed (hcp) structure. This is the first time to
directly synthesize hcp Au nanostructures by the wet chemical
method.
Besides the single-phased metal structures, Pt-on-Pd bimetallic
nanodendrites on polyvinylpyrrolidone (PVP)-modified rGO
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sheets have been synthesized in a seed-mediated sequential
process, where the Pd seeds were synthesized on rGO by the
reduction of H2PdCl4 with HCOOH, and then Pt nano-
dendrites on the Pd seeds were formed by reducing K2PtCl4with ascorbic acid (Fig. 1B).186 In this work, the pre-synthetic
functionalization of rGO with PVP has provided good
solubility of the rGO sheets in aqueous solution, and high
nucleation density of the Pd seeds.
As an alternative, the photochemical reduction has also
been applied for synthesis of graphenemetal nanostructure
composites.114,187 For example, our group has synthesized the
fluorescent Au nanodots (NDs, with sizeo2 nm) on thiol-modified
GO/rGO surfaces by simply in situ reducing HAuCl4 under
light irradiation.114 Especially, if octadecylthiol (ODT) is used
to modify the rGO surfaces, the in situ synthesized Au NDs
self-assemble into short ND-chains on the ODT-rGO surfaces
(Fig. 1C), along the h100i direction of the graphitic lattice.
This is due to the specific pattern formation of the long chain
thiol molecules on the rGO surface, which in turn serves as a
secondary template for the synthesis and self-assembly of Au
NDs. In another approach, semiconductor nanoparticles
loaded on GO/rGO sheets are employed to catalyze the
reduction of metal ions. The excited state interaction
between semiconductor and GO/rGO has been studied in
Table 2 Comparison of typical synthetic methods for grapheneinorganic nanostructure composites and their related applications
Materials Synthetic routes Applications Ref.
Au/GO or rGO Ex situ: pp stacking via 2-mercaptopyridine Catalysis, SERS 145Ex situ: pp stacking via bovine serum albumin 146In situ: photochemical reduction 114In situ: reduction by hydroxyl-amine Raman enhancement 142In situ: sonolytic reduction in poly(ethylene glycol) at 211 kHz 127
Au/rGO In situ: reduction by NaBH4 Plasmonics 35
In situ: reduction by amino terminated ionic liquid; perylene-modified rGO Electrocatalysis 139In situ: reduction by sodium citrate SERS 144In situ: reduction by ascorbic acid in the presence of CTAB Raman enhancement 185In situ: microwave assisted reduction 128,190
Au/pristine graphene In situ: thermal evaporation Identification of layer numberof pristine graphene
189
Ag/GO or rGO Ex situ: pp stacking via bovine serum albumin 146In situ: reduction by NaBH4 150In situ: electroless deposition SERS 20
Ag/TiO2/rGO In situ: Ag is reduced by the photo-generated electrons from TiO2/rGO 187Ag/rGO In situ: microwave assisted reduction 128Pt or Pd/GO or rGO Ex situ: pp stacking via bovine serum albumin 146Pd/rGO In situ: microwave assisted reduction 128Pt-on-Pd/rGO In situ: sequential reduction of H2PdCl4 with HCOOH, and K2PtCl4
with ascorbic acidEletrocatalysis 186
Cu/rGO In situ: microwave assisted reduction 128Ru/rGO In situ: microwave assisted reduction Catalysis 155Rh/rGO In situ: microwave assisted reduction Catalysis 155CdS/rGO Ex situ: pp stacking via benzyl mercaptan Optoelectronics 179TiO2/GO Ex situ: non-covalent adhesion via solution mixing of P-25 TiO2 and GO DSSC 156
Ex situ: non-covalent adhesion via solution mixing with the GO film Photocatalysis 158TiO2/rGO Ex situ: noncovalent adhesion via Nafion DSSC 161
Ex situ: self-assembly of TiO2 nanorods and rGO at two-phase interface Photocatalysis 67In situ: templated hydrolysis starting with TiCl3 and titanium isopropoxide Photocatalysis 68In situ: hydrolysis starting with TiCl3 Li ion battery 157In situ: hydrolysis starting with titanium butoxide DSSC 160In situ: hydrothermal starting with P25 and GO Photocatalysis 162
ZnO/rGO In situ: electrochemical deposition Photovoltaics 19Cu2O/rGO In situ: electrochemical deposition Photovoltaics 18Cl-doped Cu2O/rGO In situ: electrochemical deposition Optoelectronics 102Co3O4/GO or rGO Ex situ: electrostatic via aminopropyltrimethoxysilane Li ion battery 55
In situ: reduction of Co(OH)2/GO at 450 1C Li ion battery 54Co3O4/rGO In situ: microwave assisted reaction between urea and Co(NO3)2 Supercapacitor 170
MnO2/GO In situ: redox reaction between MnCl2 and KMnO4 Supercapacitor 168In situ: microwave assisted reaction between C and KMnO4 Supercapacitor 169
MnO2/FGS In situ: redox reaction between Na2SO4 and KMnO4 Li ion battery 166SiO2/GO or rGO Ex situ: electrostatic via aminopropyltrimethoxysilane 55SiO2/FGS In situ: hydrolysis starting with TEOS 166SnO2/FGS In situ: redox reaction starting with SnCl2 and H2O2 Li ion battery 166Fe3O4/rGO In situ: hydrolysis starting with FeCl3 Li ion battery 173Fe3O4/GO In situ: redox reaction starting with FeCl2, FeCl3 and NaOH Magnetic drug carrier 175
In situ: reaction of ferric triacetylacetonate with GO in 1-methyl-2-pyrrolidone Magnetic composite 172NiO/graphene In situ: sputtering Nanocapacitor 176RuO2/rGO In situ: redox reaction starting with RuCl3 and GO Supercapacitor 59CdS/rGO In situ: chemical bath deposition Solar cell 180CdS/rGO In situ: hydrothermal starting with dimethylsulfoxide, GO and Cd(CH3COO)2 Optoelectronics 181CdSe/CVD-graphene In situ: electrochemical deposition templated by a porous silica film Optoelectronics 183
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the GO/ZnO system, based on the photoluminescence decay
measurements.165 It is shown that the lifetime of ZnO
emission is decreased in the presence of GO sheets, indicating
the fast electron transfer from ZnO to GO with a transfer-rate
constant of 1.2 109 s1. Such rapid electron transfer between
semiconductor nanoparticles and GO/rGO makes their
composites attractive photocatalysts. As an example, after
GO sheets are photocatalytically reduced in the presence of
light-excited TiO2 nanoparticles, the resulting rGO/TiO2hybrid catalyst is subsequently used for photo-reduction of
Ag+. During this process, the photo-generated electrons from
TiO2 are rapidly captured by rGO and then transferred to
reduce the absorbed Ag+ ions on the rGO surface.187
In addition to metals, metal oxides, such as RuO2,54 SnO2
166
and MnO2,168 can also form on GO/rGO surfaces via in situ
reduction/oxidation. For example, GO/RuCl3
was heated with
a hot soldering iron in a N2 atmosphere and in situ reduced to
rGO/RuO2 nanocomposites,54 and KMnO4 reacting with
MnCl24H2O in the presence of GO sheets has led to
MnO2GO composites.168
Microwave irradiation is a rapid and facile method to
provide energy for chemical reactions. It can be used for the
simultaneous formation of metal NPs (e.g. Cu, Pd, Au
and Ag)128 and reduction of GO. Additionally, the in situ
microwave irradiation has been used to prepare metal oxide
rGO hybrids, such as rGOMnO2 NPs169 and rGOCo3O4
NPs.170 In the former work, rGO serves as the carbon source
to react with KMnO4 under microwave irradiation to form
CO23 , HCO
3 , and MnO2.169 In spite of the ease of process
and scalable production, the aforementioned microwave-
assisted syntheses did not show the fine control over the size
uniformity and surface distribution of NPs on rGO surfaces.
More recently, the microwave irradiation was combined with
ironic liquid (IL)-assisted dispersion of rGO to make Ru/rGO
and Rh/rGO composites, which exhibit densely loaded Ru or
Rh NPs with narrow size distributions of 2.20.4 nm and
2.80.5 nm, respectively.155
4.2.2 Electroless deposition. The electroless deposition of
metals has been previously observed on single-walled carbon
nanotubes (SWCNTs), where Au and Pt NPs formed on the
side-walls of SWCNTs immediately after immersing the
SWCNTs in solution of HAuCl4 or Na2PtCl2.188 Because
the Femi level of SWCNT is higher (less negative) than the
redox potential of Au or Pt, the SWCNT serves as the
cathode to donate electrons for the nucleation of metal
NPs. Based on the similar mechanism, we have shown that GO
can be used to template the growth of Ag nanostructures via
the electroless deposition, by simply heating GO films
deposited on APTES-modified Si/SiOx substrates in AgNO3solutions.20 In addition to GO, rGO sheets can also lead to the
formation of Ag NPs due to the presence of many aromatic
conjugated domains for electron donation. Importantly, the
abundant residual oxygen-containing groups on GO provide
more nucleation sites for metal nucleation, thus resulting in
formation of smaller Ag NPs with higher density compared to
using rGO as the template in the electroless deposition.
4.2.3 Solgel methods. The solgel process is a popular
approach for preparation of metal oxide structures and film
coatings, with the metal alkoxides or chlorides as precursors
that undergo a series of hydrolysis and polycondensation
reactions. It has been used to in situ prepare TiO2,68,157,160
Fe3O4173 and SiO2
166 nanostructures on FGS (or rGO) sheets.
Taking TiO2 as an example, the typical precursors used are
TiCl3,157 titanium isopropoxide,68 and titanium butoxide,160
which have resulted in nanorods, NPs, or macromesoporous
framework of TiO2 depending on the different experimental
conditions applied. The key advantage of the in situ solgel
process lies in the fact that the surface OH groups of the
GO/rGO sheets act as the nucleation sites for the hydrolysis,
so that the resulting metal oxide nanostructures are chemically
bonded to the GO/rGO surfaces.
4.2.4 Hydrothermal methods. Hydrothermal is a powerful
tool for synthesis of inorganic nanocrystals, which operates at
elevated temperatures in a confined volume to generate high
pressure. The one-pot hydrothermal process can give rise to
nanostructures with high crystallinity without post-synthetic
annealing or calcination, and at the same time reduce GO
to rGO. Typically, TiO2rGO composites162 and CdSrGO
composites have been prepared by the hydrothermal process.
For example, after the mixture of GO and Cd(CH3COO)2in dimethylsulfoxide (DMSO) was heated in an autoclave at
180 1C for 12 hours,181 the simultaneous reduction of GO to
rGO and the formation of CdS NPs were realized, with the
DMSO acting as both the solvent and the sulfur source.
Fig. 1 (A) TEM image of Au square sheets synthesized on GO.
Reproduced from ref. 38. (B) TEM image of solution-synthesized
Pt-on-Pd nanodendrites on rGO. Reproduced with permission from
ref. 186. Copyright 2009, American Chemical Society. (C) TEM image
of ordered Au NDs photochemically synthesized on rGO. Reproduced
with permission from ref. 114. Copyright 2010, John Wiley & Sons,
Inc. (D) TEM image of ordered CdSe NPs electrochemically deposited
on a graphene sheet. Reproduced with permission from ref. 183.
Copyright 2010, John Wiley & Sons, Inc.
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4.2.5 Electrochemical deposition. Direct electrochemical
deposition of inorganic crystals on graphene-based substrates,
without the requirement for post-synthetic transfer of the
composite materials, is an attractive approach for thin film-
based applications. Nanostructures of ZnO, Cu2O and
CdSe18,19,183 have been successfully deposited on rGO or
CVD-graphene films. As an example, ZnO nanorods have
been deposited on spin-coated rGO thin films on quartz, by
using oxygen saturated aqueous solution of ZnCl2 and KCl as
the electrolyte.19 This method has also been extended to
deposit Cu2O nanostructures18 as well as Cl-doped n-type
Cu2O102 on rGO films, spin-coated on flexible polyethylene
terephthalate (PET) substrates. In addition to the random
deposition of nanostructures on rGO films, ordered nano-
structure patterns can also be prepared by electrochemical
deposition using porous template.183 As reported, a layer of
mesoporous silica film was first formed on the CVD-grown
graphene sheets via a solgel process, and then CdSe NPs were
electrochemically deposited on the graphene surface through
the pores of the pre-coated silica film. After removing the silica
film by HF etching, ordered CdSe NPs on graphene are
revealed (Fig. 1D).
4.2.6 Thermal evaporation. The thermal evaporation has
been utilized to deposit Au NPs on the pristine graphene and
how the layer number of graphene affects the particle size and
density of deposited Au NPs has been studied.189 It is found
that the particle density increases and size decreases with
increasing the layer number of graphene. This interesting
phenomenon has been attributed to two factors. First, the
diffusion coefficient of deposited Au atoms varies on different
surfaces, which governs the nucleation and growth of the Au
islands. Second, the surface free energy of graphene depends
on the layer number, which controls the interaction between
graphene and the evaporated Au atoms, and in turn affects the
surface absorption, desorption and diffusion of the Au atoms
on graphene surface.189
4.2.7 Ordered metal oxidegraphene composites via in situ
self-assembly. In contrast to the random stacking of graphene-
based hybrid nanosheets, a novel method has been developed
to prepare the ordered metal oxidegraphene hybrids by the
surfactant assisted self-assembly.166 In this work, the anionic
surfactants were first mixed with rGO sheets, which attached
to the hydrophobic domains of the surfactant micelles
(Fig. 2A). After that, metal cations were introduced and
bonded to the surfactants assembled on rGO, to give an
ordered overall structure (Fig. 2B). In situ crystallization then
took place, via redox or hydrolysis reactions, to result in the
alternating layers of rGO/metal oxides, e.g. NiO, SnO2, and
MnO2 (Fig. 2C and D). This assembly process is important
in constructing layered composite materials for lithium ion
batteries, which will be discussed in section 7.1.
5. Graphenepolymer composites
The graphenepolymer composites, based on the 3D arrange-
ment and the kind of interaction between graphene sheets and
polymers, can be classified into three types, i.e. graphene-filled
polymer composites, layered graphenepolymer films, and
polymer-functionalized graphene nanosheets.
5.1 Graphene-filled polymer composites
Carbon-based materials, such as amorphous carbon and
carbon nanotubes (CNTs), are conventional fillers for enhancing
the electronic, mechanical and thermal properties of polymer
matrices. CNT has been regarded as one of the most effective
filler materials, but with relatively high cost. Graphene-based
fillers are thus expected as a promising replacement or supple-
ment to CNTs. In order to lower the content of the graphene
filler, the dispersity and its bonding with the polymer matrix are
important factors to achieve optimal properties of the compo-
sites. With this consideration in mind, graphene-filled polymer
composites are commonly prepared by solution mixing,42,191195
melt blending,196198 and in situ polymerization.199210
5.1.1 Fabrication methods. Solution mixing is the most
straightforward method for preparation of polymer compo-
sites. The solvent compatibility of the polymer and the filler is
critical in achieving good dispersity. Due to the residual
oxygen-containing functional groups, GO can be directly
mixed with water soluble polymers, such as poly(vinyl alcohol)
(PVA), at various concentrations.191,211 However, GO does
not dissolve in non-polar solvents, and other forms of
graphene such as expanded graphite (EG) and rGO show
limited solubility in both organic and inorganic solvents. In
order to overcome this problem, sonication has been used to
Fig. 2 (AC) Schematic illustrations of the self-assembly approach to
prepare ordered metal oxide/rGO composites. (A) Adsorption of
surfactant hemimicelles on the surfaces of the rGO or rGO stacks
causes its dispersion in surfactant micelles in an aqueous solution. (B)
The self-assembly of anionic sulfonate surfactant on the graphene
surface with oppositely charged metal cation (e.g. Sn2+) species and
the transition into the lamella mesophase toward the formation ofSnO2 graphene nanocomposites, where hydrophobic rGO sheets are
sandwiched in the hydrophobic domains of the anionic surfactant. (C)
Layered metal oxiderGO composites composed of alternating layers
of metal oxide nanocrystals and rGO stacks after crystallization of
metal oxide and removal of the surfactant. (D) High-magnification
TEM of SnO2/rGO composites. The layered structure of SnO2 is
composed of connected nanocrystalline SnO2 with 45 nm diameter
interspaced by rGO stacks. Reproduced with permission from ref. 166.
Copyright 2010, American Chemical Society.
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produce metastable dispersions of graphene derivatives, which
are then mixed with polymer solutions, such as those of
poly(methyl methacrylate) (PMMA),192 polyaniline (PANi),195
polycaprolactone (PCL),193 and polyurethane (PU).194,212
High-speed shearing combined with ice-cooling has also been
applied to mix graphene-based fillers and the polymer
matrices.42 However, in the two approaches mentioned above,
re-stacking, aggregation and folding of the graphene based
nanosheets are unavoidable during the process, which signifi-
cantly reduce the specific surface area of the 2D fillers. Thus,
surface functionalization of graphene-based fillers before
solution mixing must be carried out to provide them with
good solubility in various kinds of solvents. As an example,
phenyl isocyanate-modified GO sheets have shown improved
dispersity in polystyrene (PS) solution in DMF. During the
subsequent in situ reduction of GO, the polymer matrix
prevents the re-aggregation of rGO sheets to retain a homo-
geneous suspension.138
Melt compounding utilizes both high-shear forces and high-
temperature melting to blend the filler and matrix materials.
Hence, it does not require a common solvent for the graphene
filler and the polymer matrix. Polylactide (PLA)-exfoliated
graphite (EG) composite 196 and PETrGO graphene composite197
were successfully prepared by using melt compounding.
However, the high shear forces employed in melt compounding
can sometimes result in the breakage of the filler materials,
such as CNTs and graphene nanosheets.198
In situ polymerization is another often used method to
prepare graphene-filled polymer composites, such as those
with epoxy199201,213 and polyaniline (PANi).202206 In a
typical process for preparation of grapheneepoxy composite,
after the graphene based filler is mixed with epoxy resins under
high-shear forces, a curing agent is added to initiate the
polymerization.199 The polymerization for PANi, on the
other hand, is an oxidative process, thus an oxidative agent,
such as ammonium persulfate, is used to facilitate the
polymerization.204 Alternatively, graphenePNAi composite
can be prepared by the in situ anodic electropolymerization.207
For example, PANi has been directly deposited on the
working electrode made from graphene paper in a three-
electrode cell, with a solution containing aniline monomers
as the electrolyte. In addition to the epoxy and PANi,
graphene has been successfully incorporated in other polymer
matrices, such as silicone,208 PS209 and poly(vinyl chloride/
vinyl acetate) copolymer,210 via the in situ polymerization.
5.1.2 Property enhancement. Electrical conductivity has
shown significant improvement in graphene-filled polymers.
Very low percolation thresholds have been achieved in various
types of insulating matrices, for example, 0.15 vol% loading of
rGO in poly(vinyl chloride/vinyl acetate) copolymergraphene
composites,210 0.47 vol% filler loading in PETgraphene
composites (with a conductivity of 2.11 S m1 obtained at
a loading of 3.0 vol%),197 and 0.1 vol% filler loading in
PS-functionalized graphene sheets (FGS) composites.138
The effective property enhancement in these reports has
been attributed to the large specific surface area and the
p-conjugated 2D conducting surface of graphene-based
sheets, their homogeneous dispersion in the polymer matrix,
and the fillermatrix interaction induced by the surface func-
tional groups.
Besides the electrical conductivity, graphene-based fillers
can improve the mechanical strength of the polymer as well,
since the monolayer graphene is one of the strongest materials
with a Youngs modulus of 1.0 TPa and a breaking strength of
42 N m1.4 By using solution mixing and vacuum filtration,
the strong and ductile poly(vinyl alcohol)(PVA)GO compo-
site paper has been prepared, which shows a Youngs modulus
of 4.8 GPa and a tensile yield strength ofB110 MPa with
3 wt% of the GO loading. Unfortunately, in the physically
mixed composite, the relative movement between the filler and
matrix cannot be avoided under external stresses, which limits
the achievable maximum strength. To alleviate or solve this
problem, it is necessary to chemically tailor the structure at the
filler/matrix interface to facilitate the efficient load transfer.
For example, GO filler was covalently bonded to isocyanated-
PU matrix via the reaction between the oxygenated groups of
GO and the isocyanate groups at the end of PU chains. This
chemical bonding has led to the increase in the Youngs
modulus and hardness byB900% andB327%, respectively.194
The thermal stability is another important property for
functional polymers, which can be improved by embedding
materials with superior thermal properties like the graphene-
based fillers. For example, FGSPMMA composite gives an
increase of 30 1C in Tg (glass transition temperature, above
which chains of thermoplastic polymer begin to flow) with a
loading of 0.05 wt% FGS, whereas the Tg of poly(acrylonitrile)
(PAN)FGS composite increases more than 40 1C with a
loading of 1 wt% FGS,42 which surpasses all other carbon-
based nanofillers, such as the expanded graphite (EG) and the
graphitic nanoplatelets (GNPs).192 The superior performance
of FGSs fillers, arises from the presence of larger amount of
monolayer graphene sheets with wrinkled morphology and
functionalized surfaces, which benefits the strong fillermatrix
interaction (Fig. 3A and B).42 Besides the thermoplastics,
graphene-filled elastomers also exhibit enhanced thermal
stability in terms of increased degradation temperature.
Examples include the siliconeFGS composites with a 55 1C
increase in the degradation temperature with 0.25 wt%
loading of the filler,208 and the PLAEG composite with the
degradation temperature improved by 10 1C with a loading of
only 0.5 wt% of EG.196
Furthermore, the graphene-based filler materials have also
enhanced the thermal conductivity of polymer matrices, provided
that the single layer graphene has a thermal conductivity
of up toB5000 W mK1 at room temperature.214 When only
Fig. 3 SEM images of (A) EGPMMA and (B) FGSPMMA showing
fracture surface topography. Reproduced with permission from
ref. 42. Copyright 2008, Nature Publishing Group.
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0.25 wt% of FGSs were incorporated in the silicon foam
matrix, the increase of 6% in the thermal conductivity was
obtained.208 In addition, epoxy with embedded few-layer
graphene sheets (B25 vol%, layer number of B4) has
displayed an increase in the thermal conductivity of more
than 30 times at k = 6.44 W mK1.199 This result outperforms
many conventional fillers, such as Al, Ag and SiO2, which
require more than 50% loading to achieve the similar result.
5.2 Layered graphenepolymer films
In contrast to the graphene-filled polymer composites, where
the graphene fillers are randomly distributed in the polymer
matrices, graphene derivatives have also been composited with
polymers in layered structures, which are fabricated for
specific applications, such as the directional load-bearing
membranes, and thin films for photovoltaic applications.
For example, layer-by-layer (LbL) assembling via the
LangmuirBlodgett (LB) technique has been used to deposit
GO sheets onto films of polyelectrolyte poly(allylamine
hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate)
(PSS) multi-layers (Fig. 4A).215 The resulting compositedmembrane shows enhanced directional elastic modulus by an
order of magnitude, i.e. from 1.5 to 20 GPa with 8 vol%
loading of the graphene (Fig. 4B). The similar strategy has
been used to prepare multi-layer PVAGO films, which also
display improved elastic modulus and hardness.216 Another
application that requires layered graphenepolymer compo-
sites is in polymer-based photovoltaic devices.217219 These
composite films are fabricated by the sequential spin-coating
of functional components in the device configurations. For
example, GO film and poly(3-hexylthiophene) (P3HT)/phenyl-
C61-butyric acid (PCBM) blends are deposited layer after
layer on ITO substrate, where the GO layer acts as the hole
transport segment in the photovoltaic device.218
5.3 Polymer functionalized graphene nanosheets
Instead of being used as fillers to enhance the properties of
polymers, graphene derivatives can be applied as 2D templates
for polymer decoration via covalent and non-covalent functio-
nalizations. The polymer coating, on the other hand, assists to
improve the solubility of the graphene derivatives, and offers
additional functionality to the resulting hybrid nanosheets.
The covalent functionalization of graphene derivatives is
mainly based on the reaction between the functional groups
of the polymers and the oxygenated groups on the GO or rGO
surfaces. Esterification of the carboxylic groups in GO with the
hydroxyl groups in PVA has been demonstrated in the synthesis
of GOPVA composite sheets.220 The carboxylic groups on GO
were also involved in the carbodiimide-catalyzed amide forma-
tion process to bind with the six-armed polyethylene glycol
(PEG)-amine stars.221 However, the carboxylic groups are
mainly confined at the periphery of the GO sheets. In addition,
the grafting of certain polymers requires the presence of
non-oxygenated functional groups on GO, such as amine and
chloride groups. Therefore, alternative strategies with addi-
tional chemical reactions have to be developed to modify GO/
rGO surfaces with desired functional groups before the grafting
of polymers. For example, after 4-bromophenyl groups were
coupled on the rGO surface through the diazonium reaction, a
fluorenethiophenebenzothiadazole polymer was covalently
grafted to the rGO surface through the Suzuki reaction via
the 4-bromophenyl groups.222 The pre-bonded diazonium
group on rGO can also act as the initiator for atomic transfer
radical polymerization (ATRP), based on which, PS has been
successfully grafted onto the rGO sheet with controlled
density.223 Additionally, acyl-chlorinated GO sheets can further
react and connect to triphenylamine-based polyazomethine
(TPAPAM)224 and MeOH-terminated P3HT,225 and the
APTES-modified GO surface can be further bonded with
maleic anhydride-grafted polyethylene (MA-g-PE).226
As shown in the aforementioned examples, the covalent
functionalization of polymers on graphene-based sheets holds
versatile possibility due to the rich surface chemistry of
GO/rGO. Nevertheless, the non-covalent functionalization,
which relies on the van der Waals force, electrostatic inter-
action or pp stacking,227 is easier to carry out without
altering the chemical structure of the capped rGO sheets,
and provides effective means to tailor the electronic/optical
property and solubility of the nanosheets. The first example of
non-covalent functionalization of rGO sheets was demonstrated
by the in situ reduction of GO with hydrazine in the presence
Fig. 4 (A) Schematic illustration of fabrication and assembly of the free-standing GO-LbL film. (B) Plot showing the variation of elastic modulus
calculated theoretically (under parallel and random orientation) and that obtained experimentally (using buckling and bulging measurements) with
the volume fraction of GO. Reproduced with permission from ref. 215. Copyright 2010, American Chemical Society.
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of poly(sodium 4-styrenesulfonate) (PSS),228 in which the
hydrophobic backbone of PSS stabilizes the rGO, and the
hydrophilic sulfonate side groups maintains a good dispersion
of the hybrid nanosheets in water. Later on, conjugated
polyelectrolytes with various functionalities have been usedto modify rGO nanosheets,14,17,41 in the hope to achieve good
solubility in different kinds of solvents, and at the same time
acquire added optoelectronic properties. Our group has
specially designed an amphiphilic coilrodcoil conjugated
triblock copolymer (PEGOPE, chemical structure shown in
Fig. 5A) to improve the solubility of graphenepolymer
nanocomposites in both high and low polar solvents.14 In
the proposed configuration, the conjugated rigid-rod back-
bone of PEGOPE can bind to the basal plane of the in situ
reduced GO via the pp interaction (Fig. 5B), whereas the
lipophilic side chains and two hydrophilic coils of the back-
bone form an amphiphilic outer-layer surrounding the rGO
sheet. As a result, the obtained rGO sheets with a uniformlycoated polymer layer (Fig. 5C) are soluble in both organic low
polar (such as toluene and chloroform) and water-miscible
high polar solvents (such as water and ethanol).
6. Other graphene-based composites
Other than inorganic nanostructures and polymers, materials
such as organic crystals,39,40 metalorganic frameworks
(MOF),4345 biomaterials,4648 and carbon nanotubes (CNTs)5053 have also been composited with graphene derivatives for
various applications.
For example, N,N0-dioctyl-3,4,9,10-perylenedicarboximide
(PDI)graphene core/shell nanowires have been prepared
through the pp interaction, and used in organic solar cells.39
Besides, peptidegraphene core/shell nanowires were prepared
by mixing graphene aqueous solution and the organic peptide
solution (diphenylalanine in 1,1,1,3,3,3-hexafluoro-2-propanol).
After removal of peptide cores by calcination, the hollow
graphene tubes were applied as supercapacitor electrodes.40
MOF, a recently emerging material, is promising in a
number of gas purification and storage based applications,
and has also been composited with GO/rGO sheets.4345 As an
interesting example, benzoic acid-functionalized graphene has
been found to act as a structure-directing template for the
growth of MOF-5, which exhibits the wire-like structure along
its [220] direction. Along the long axis of the wire, the
functionalized graphene sheets are periodically distributed,
which are expected to impart photoelectric transport property
to the insulating structure of MOF-5.43
Moreover, biomaterials like DNA hybridized with GO or rGO
are used in fluorescent sensing platforms based on the fluores-
cence resonance energy transfer (FRET),46,47 and biocompatible
aptamerGO composites can be applied in probing living cells.48
GrapheneCNT composites have also been prepared via solution
blending50,51 or in situ CVD growth53 to be applied in Li ion
batteries,51 transparent conductors,50 and supercapacitors.52,53
7. Applications of graphene-based composites
7.1 Lithium ion batteries
The increasing demand from the current information-rich
society to provide high efficient, low cost and green solutions
for energy conversion and storage devices, has been constantly
driving the development of battery systems, in particular, the
rechargeable batteries. Li ion battery (LIB) is considered as
one of the most promising storage systems, because of itshigh absolute potential against the standard hydrogen cell
(3.04 V) and its low atomic weight (M = 6.94 g mol1),
which leads to the large energy density with a theoretical value
of up to B400 Wh kg1.229,230 In addition to the traditional
insertion-type (e.g. TiS2 and LiCoO2)231,232 and alloying-type
(e.g. Sn)233 of electrode systems for insertion/extraction of Li,
the conversion-type LIB has been intensively studied recently,
which employs transition metal compounds (MaXb, M = Co,
Ni, Fe, Cu etc.; X = O, S, P , N etc.) to facilitate the Li
insertion/extraction through redox reactions (reaction (1))
between the ionic and metallic states of the metal, thus
providing high capacities.234
MaXb + cLi+ + ce2 LicXb + aM (1)
7.1.1 Graphenemetal oxide nanostructures for LIBs. The
nanostructured MaXb has been proven to be advantageous
compared to micrometre-sized structures for LIBs, owing to
the shortened diffusion length for both Li ions and electrons,
and larger specific surface area for the electrode/electrolyte
interaction.235 However, drawbacks like poor conductivity,
low packing density thus reduced volumetric energy density, as
well as the expansion induced lose of contact points, remain
problematic and require continuous development. Although
graphene and its derivatives cannot effectively host Li via
intercalation like in bulk graphite, they are able to store Li
through the surface absorption and functional groups induced
bonding,236 and they possess high conductivity and large
surface area. Therefore, many metal oxide nanostructures, like
SnO2,166,167,237 Co3O4,
54,55 MnO2,56 TiO2,
157,238 Fe3O4173 and
Cu2O,177 have been composited with graphene for LIBs.
Rapid capacity decay is usually observed in the pure metal
oxide based anodes, due to problems like poor conductivity,
structure degradation and expansion, and inter-particle
agglomeration. Using graphene based materials as matrices for
in situ synthesis and anchoring of the metal oxide nanostructures
is anticipated to solve these problems. First of all, the incorpora-
tion of graphene maintains a good conducting network in the
Fig. 5 (A) Chemical structure of PEGOPE. (B) Schematic illustra-
tion of fabrication of PEGOPE stabilized rGO sheets. (C) Tapping-
mode AFM image and cross-sectional analysis of PEGOPErGO on
mica. Reproduced with permission from ref. 14. Copyright 2010,
John Wiley & Sons, Inc.
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hybrid electrode. This effect has been elucidated in a recent report
on the Mn3O4/graphene based anode.56 Although Mn3O4
is an attractive anode material for LIBs considering its low
cost, abundance, and high theoretically value in capacity
(B936 mAh g1), its low conductivity (B 107108 S cm1)
has limited its practical application with a measured capacity of
400 mAh g1.56 Direct growth of Mn3O4 NPs on graphene
sheets provides a good contact between the NPs and the 3D
network of graphene, thus realizes efficient conduction of charge
carriers. Consequently, the graphene/Mn3O4 hybrid anode
affords a specific capacity of B900 mAh g1, about two times
higher than that by using the pure Mn3O4. The same effect,
arising from the conducting network of graphene sheets, has also
been demonstrated in SnO2/graphene,237 TiO2/graphene
157 and
Fe3O4/graphene173 electrodes.
By using graphene as templates, another attractive aspect is
that the synthesized NPs can be evenly distributed on the
graphene surfaces. In this way, the agglomeration of NPs can
be considerably reduced and the large active surfaces of the
NPs are able to participate in Li/electron diffusion more
efficiently during the discharge/charge cycles. As a result,
enhanced specific capacity and cycling performance have
been achieved in systems like Co3O4/graphene54 and Cu2O/
graphene177 composites. In addition, the elastic and flexible
graphene sheets can accommodate the volume expansion of
the NPs upon Li insertion/extraction; whereas the NPs in turn
prevent the re-stacking of the graphene layers.173 More
importantly, it has been shown that the structure integrity of
the crystalline NPs on graphene can be maintained after many
cycles of discharge/charge. As an example, the Li insertion/
extraction in SnO2 based anode usually involves a two phase
process, i.e. in the first cycle, the SnO2 reacts with Li+ to give
Sn and Li2O; after that, Sn will not reverse back to SnO2 but
form SnLix alloy for the rest cycles,237,239 which leads to large
volume change and rapid capacity decay. This problem can be
overcome by using SnO2/graphene hybrid anode, which shows
a well maintained SnO2 crystalline structure after 50 cycles.167
Although the graphene-based 2D templates have provided
the good NP distribution on individual sheets, the graphene
NP hybrid sheets are randomly stacked to make electrodes,
inevitably leading to the particleparticle aggregation. Thus,
the ordered graphenemetal oxide composites166 and graphene
encapsulated metal oxide nanostructures55 have been developed
to further reduce the particle aggregation. In the former case,
layers of graphene stacks and metal oxide NPs (e.g. SnO2) are
assembled alternatively through a surfactant assisted self-
assembly process (Fig. 2 in section 4.2.7). Such highly ordered
and stable structure of graphene/SnO2 exhibits a specific
energy density of B760 mAh g1, which is comparable to
the theoretical value for SnO2 without significant charge/
discharge degradation.166 However, most graphene stacks
consist of multi-layer graphenes, which reduce the useful
surface area of graphene and increase the carbon content in
the composite. In an alternative approach, graphene sheets are
made to wrap around NPs of SiO2 or Co3O4 via the electrostatic
interaction in the graphene encapsulated-metal oxide hybrids
(Fig. 6A).55 The graphene encapsulation is able to separate
individual NPs from one another, which prevents the particle
agglomeration, and maintains a conducting network to
effectively connect all NPs. The resulting graphene/Co3O4 based
anode, with a low carbon content ofo8.5 wt%, displays a very
high capacity of 1100 mAh g1 in the first 10 cycles, and
maintains at above 1000 mAh g1 even after 130 cycles (Fig. 6B).
7.1.2 Graphenecarbon nanotube or graphenefullerene
hybrid for LIB. To date, besides the graphenemetal oxide
based anode materials for LIBs, graphenecarbon nanotube
(CNT) or graphenefullerene (C60) hybrid electrodes are also
reported.51 The graphene nanosheets (GNSs), prepared from
reduction and re-assembly of oxidized GO sheets, consist of
615 re-stacked individual graphene monolayers. The GNSs
show a specific capacity ofB540 mAh g1, which is much
larger than that of graphite due to the enlarged specific surface
area after exfoliation. The energy storage property of this
electrode can be further improved by mixing GNSs with
nanocarbons, such as CNTs and C60, to increase the spacings
among the re-stacked GNSs. In this way, the additional voidsfor Li insertion/extraction are created in the mixtures, resulting
in B40% increase in the specific capacity.
7.2 Supercapacitors
Supercapacitor, or ultracapacitor, is another type of electro-
chemical energy storage device, which provides high power
density (10 kW kg1), short charge/discharge time (in seconds),
and long cycling life, as compared to the battery device.240
Thus, supercapacitor is widely applied for powering the heavy
vehicles, portable media players, personal computer (PC)
cards, and so on. Generally, there are two types of
Fig. 6 (A) Schematic illustration of fabrication of graphene-
encapsulated metal oxide NPs. (B) Cycle performance of graphene-
encapsulated Co3O4. Reproduced with permission from ref. 55.
Copyright 2010, John Wiley & Sons, Inc.
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supercapacitors, in terms of their operation mechanisms.241 One
is the electrical double layer capacitor (EDLC), which stores
energy via an electrostatic process, i.e. the charges are accumu-
lated at the electrode/electrolyte interface through polarization.
Hence, it is essentially important to use electrode materials with
good conductivity and large specific surface areas in EDLCs,
such as the activated carbon, CNTs, carbon nanofibers, and the
emerging graphene-based 2D sheets. The graphene-based
materials, in particular rGO, are advantageous in terms of the
chemically active surface with large specific area, good con-
ductivity, low cost, and mass production with solution-based
processability as mentioned before. In addition, it is suggested
that the aggregated graphene sheets exhibit an open-pore
structure, which allows for the easy access of electrolyte ions
to form electric double layers.242 The first EDLC based on the
chemically modified graphene, or rGO, was demonstrated by
Ruoff and coworkers.2 A specific capacitance of 135 F g1 in
the aqueous electrolyte is obtained, which is comparable with
the traditional carbon-based electrode materials.
The other type of supercapacitor is so called the pseudo-
capacitor, which is based on the rapid redox reactions of the
chemical species present in the electrode.241 Commonly used
electrode materials are metal oxides (e.g. RuO2, NiO and
MnO2) and conducting polymers (e.g. Polyaniline, PANi and
Polypryrrole, PPy). This type of electrode affords higher
specific capacitance per unit surface area (1 to 5 F m2)
compared to the porous carbon based EDL electrode (0.1 to
0. 2 F m2).242 However, the relatively high cost and low
conductivity of metal oxides or conducting polymers have
limited their applications. As a result, graphene derivatives
and metal oxides or conducting polymers are combined and
used as the hybrid type of supercapacitor (i.e. the combination
of the EDLC and pseudo-capacitor).
7.2.1 Graphenemetal oxide supercapacitors. A number of
metal oxides, such as ZnO,57 SnO2,58 Co3O4,
170 MnO2168,169
and RuO2,59 have been composited with graphene derivatives
to prepare electrodes for supercapacitors. Taking the
graphene/MnO2 hybrid electrode for example,169 the MnO2
NPs contribute to the energy storage via the redox reaction in
between the III and IV oxidation states of Mn (reaction (2)),
which involves the intercalation of alkali metal ions present in
the electrolyte, e.g. Na+. The rGO sheets, on the other hand,
provide the capacitance through the electron double layer
at the carbon surface, and meanwhile afford a conducting
network for the anchored MnO2 NPs and large surface area
for the NP/electrolyte interaction, which leads to high pseudo-
capacity with rapid chargedischarge rate. The resulting
composite electrode shows a high specific capacitance of
310 F g1 at 2 mV s1 (228 F g1 at even 500 mV s1), about
3 times higher than that given by the pure rGO or MnO2.
Similarly, a graphene nanosheet (GNS)/Co3O4 supercapacitor
displays a maximum specific capacity of 243.2 F g2 at a scan
rate of 10 mV s1 in KOH aqueous solution, and B95.6% of
the specific capacitance is retained after 2000 cycles.170
MnO2 + C+ + e2 MnOOC (2)
where C+ = Na+, K+ or Li+
Compared to batteries, supercapacitors exhibit higher
power density but lower energy density. In order to improve
the energy density of a supercapacitor, while maintaining the
high power density from the graphene/metal oxide hybrid elec-
trode, an asymmetric capacitor system was employed by Cheng
and coworkers.243 In such a capacitor, graphene is incorporated
as the negative electrode (a battery type) and MnO2
nanowire/
graphene composite (MGC) as the positive electrode (Fig. 7A).
A high operating voltage (B2.0 V) is thus made possible in this
asymmetric set-up to give the high energy density (note that the
energy is directly proportional to the square of the operating
voltage across the cell). As a result, a maximum energy density of
30.4 Wh kg1 for this asymmetric capacitor is achieved, which is
much higher than the results obtained from the symmetric one,
i.e. with graphene (or graphene/MnO2) as both the positive and
negative electrodes (Fig. 7B).
7.2.2 Graphene-conducting polymer supercapacitors. Con-
ducting polymers are attractive materials for supercapacitor
electrodes, because they have moderate conductivity, e.g.
0.15 S cm1 for doped polyaniline (PANi),244 fast chargedischarge kinetics and dopingundoping processes,245
and flexibility for thin film based fabrications.246 The power
conversion and storage process in polymer-based capacitors
involves the dopingundoping redox reactions, as shown in
reactions (3) and (4):
Cp + nA2 Cpn+(A)n + ne (p-doping) (3)
Cp + ne + nC+2 Cpn(C+)n (n-doping) (4)
where Cp = conducting polymer; C+ = cation; A = anion.
Fig. 7 (A) Schematic of the assembled structure of asymmetric ECs based on MGC as the positive electrode and graphene as the negative
electrode. (B) Ragone plot related to energy and power densities of graphene//MGC asymmetric ECs with various voltage windows, graphene//
graphene and MGC//MGC symmetric ECs. Reproduced with permission from ref. 243. Copyright 2010, American Chemical Society.
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Till now, PANi is one of the most employed polymers
to be incorporated with GO or rGO sheets for super-
capacitors.195,202207,247,248 In the GOPANi composite, the
doping process is realized by linking the carboxyl group of GO
to the nitrogen of the PANi backbone.205,206 Since PANi is
synthesized on GO via the in situ polymerization, the oxygen-
containing functional groups of GO can facilitate the nuclea-
tion of the polymer and give rise to good surface coverage of
PANi and the strong pp interaction between the PANi
backbone and the GO surface. As reported, with 1% mass
loading of GO, the conductivity of the PANi based electrode
is improved from 2 S cm1 (for pure PANi) to 10 S cm1, and
a high specific capacitance of 531 F g1 (216 F g1 for the pure
PANi) is obtained in the potential range of 0 to 0.45 V at a
scan rate of 10 mV s1.206 Besides the enhanced conductivity
and capacitance, GO is also able to improve the cycling-
stability of the composite electrode, because the flexible GO
sheets can undertake mechanical deformation during the
chargedischarge cycles to protect the PANi polymer from
shrinkage and swelling.205,248 For example, a hierarchical
composite electrode of GOPANi can retain 92% of the
initial capacitance after 2000 cycles, whereas the initial capa-
citance of pure PANi electrode drops to 74%.248
Although the chemically modified graphene, or rGO,
exhibits much better intrinsic conductivity as compared to
GO, the rGOPANi electrode has not shown any improved
electrochemical performance compared to GOPANi at a low
GO/rGO content (o 20 wt%).195,203 This is possibly because,
the less amount of oxygenated functional groups on the
surface of rGO compared to GO has caused the poor poly-
merization and distribution of PANi nanostructures, and less
effective doping of PANi via the carboxyl groups.
7.2.3 Graphenecarbon nanotube (CNT) supercapacitors.
In graphene-based supercapacitors, re-stacking of graphene
sheets can lead to the decrease of active surface area. To solve
this problem, CNTs can be used as spacers to create nanopores
among graphene layers, and at the same time provide good
conductivity. For example, poly(ethyleneimine) (PEI)-modified
rGO sheets mixed with acid-oxidized multi-walled CNTs to
make hybrid carbon films gave an average specific capacitance
of 120 F g1 at a high scan rate of 1 V s1.52 The concept of
using CNT spacers is further employed in a three dimensional
(3D) CNT/graphene sandwich structure (CGS), in which
CNTs are grown amongst the graphene layers via the
CVD process (Fig. 8A and B).53 The resulting Brunauer
EmmettTeller (BET) surface area of the CGS is 612 m2 g1,
which is much higher than that of graphene (202 m2 g1).
A specific capacitance of 386 F g1 is thus obtained at a scan
rate of 10 mV s1. The capacitance increases for ca. 20%
compared to the initial value after 2000 cycles (Fig. 8C),
suggesting the excellent electrochemical stability of the hybrid
carbon electrode.
7.3 Fuel cells
Unlike batteries or supercapacitors, which store energy
chemically in the electrochemical cells, the fuel cell generates
electricity via reactions between a fuel (anode) and an oxidant
(cathode), which are continuously supplied from external
sources. There are several different combinations of the fuel
and oxidant, with typical examples including the hydrogen/
oxygen cell and the methanol/oxygen cell. Pt-based catalysts
are the most popular materials used for low-temperature fuel
cells, e.g. the oxidation of hydrogen, methanol, or reformate.249
Since Pt is very expensive, Pt loading must be minimized, but
the fuel cell performance cannot be compromised. Therefore,
carbon-based catalyst supports, such as carbon black, CNTs
and graphene, are used to provide good dispersity and thus
large effective surface area of the supported catalyst particles.
GraphenePt composites have been attempted in the fuel cells,
such as the methanol oxidation cells6062,250252 and the
oxygen reduction cells.63,253,254
Enhanced electrocatalytic activity in methanol oxidation
has been obtained in graphene-supported Pt catalysts as
compared to the commercial carbon black (Vulcan XC-72)
supported Pt NPs.60,61 The superior performance arises from
the large specific surface area of graphene and its high
conductivity for electron and ion transport. Graphene has
also shown advantages compared to the CNT-based catalyst
support. First, the 2D configuration of graphene sheets with
both sides exposed to the solution, leading to larger active
surface area compared to the 1D tube structures. Second, due
to the large curvature of the CNT walls, the deposited Pt
particles are sometimes aggregated, and not as uniformly
distributed as on graphene-based supports.250 Moreover, it is
demonstrated that the chemically converted graphene, or
rGO, displays better tolerance towards CO poisoning during
the methanol oxidation compared to CNTs, because the
residual oxygenated functional groups of rGO can react
and remove the carbonaceous species.250 In a recent report,
Yoo et al. found that graphene nanosheets (GNS) are able to
assist the formation of Pt clusters with a size of B0.5 nm,251
and because these clusters are well distributed on the GNS
surface, their aggregation into larger Pt particles is avoided.
Fig. 8 (A) Schematic illustration of the formation of hybrid materials
with CNTs grown in between graphene nanosheets, showing stacked
layers of GO (left), catalyst particles adhered onto layer surface after
deposition (middle), and CNTs in between graphene layers after
growth (right). (B) SEM image of the CGS sheet. (C) Variations of
specific capacitance versus the cycle number measured at a scan rate of
200 mV s1 in 6 M KOH within the potential range from 0.2 to
0.45 V (versus saturated calomel electrode (SCE)). Reproduced with
permission from ref. 53. Copyright 2010, John Wiley & Sons, Inc.
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In addition, based on a theoretical study, surface defects and
voids on the graphene surface can enhance the interaction
between the graphene and anchored Pt or Au clusters.255
Consequently, these sub-nanometre Pt clusters can remain
stable on GNS even after the heat treatment at 400 1C in an
Ar/H2 environment. The resulting PtGNS hybrid catalyst
shows unusually high activity for methanol oxidation, e.g. the
PtGNS exhibits a current density of 0.12 mA cm2 at 0.6 V
(versus reference hydrogen electrode), about 3 times higher
than that obtained with the Ptcarbon black catalyst.
Nitrogen-doped graphene sheets, with better conductivity
compared to un-doped ones,256 have also been used as catalyst
supports in fuel cells.62,63 The N-doped rGO can be prepared
by nitrogen plasma treatment of rGO,63 or thermal reduction
of GO in the presence of ammonia (NH3),62 which gives rise to
nitrogen species on the carbon basal plane, such as amino,
pyridine and graphitic type groups. The presence of the
nitrogen species results in a good coverage and dispersion of
the in situ synthesized Pt NPs compared to the un-doped rGO
surface. This advantage along with the N-doping induced
increase of the electronic conductivity256 has led to much
enhanced electrocatalytic activities of N-rGOPt catalysts.62,63
For example, in a methanol fuel cell, the N-doped rGOPt
catalyst exhibited an oxidation current of 135 mA mg1, which
is 2 times higher than that of the rGOPt catalyst.62
7.4 Photovoltaic devices
A photovoltaic device converts the solar energy into electricity.
Besides the widely commercialized Si based solar cells, cheap
and efficient alternatives, such as the polymer based solar cells,
dye (or quantum dots, QD) sensitized solar cells, have also
been intensively investigated. The following context will
discuss the incorporation of graphene into these devices.
7.4.1 Silicon based solar cells. In order to partially replace
silicon, thus reducing cost in solar cell devices, carbon based
materials have been attempted in the p-type amorphous
carbon/n-type silicon (p-AC/n-Si) heterojunctions,257 and
CNT/Si heterojunctions.258 However, amorphous carbon
usually encounters difficulties in tuning its electronic properties,
while the CNT-based thin film suffers from bundling and thus
reduces the film connectivity and conductivity. In contrast,
graphene-based films can be prepared with controlled thick-
ness, good surface continuity, and tunable properties via
doping, covalent or non-covalent functionalizations. CVD-
grown graphene sheets with sizes of tens to hundreds micro-
metres have been deposited on n-Si with 100% coverage to
make the Schottky junction solar cell,64 which shows an
efficiency (PCE, Z) of up to B1.5% with a fill factor
of B56% (FF, actual obtainable maximum power vs. the
theoretical power). In addition to its low cost, the graphene
film serves as a semitransparent electrode and an antireflection
coating for the graphene/n-Si solar cells, and helps to generate
a built-in voltage of 0.550.57 V for the effective electron-hole
separation.64,259 Although the obtained efficiency is still lower
than that of pure Si-based solar cells, it is suggested that the
efficiency can be increased in future by improving the
conductivity and transparency of the graphene film.64
Additionally, the tunable work function of the graphene layer
affords the additional advantage to the graphene/silicon solar
cells, since a desirable work function allows for the efficient
carrier injection at the interface. One way to adjust the work
function of the graphene film is through controlling the layer
number of the graphene sheets via the LbL deposition,260
which has shown a sequential increase in work function with
increasing layer numbers. Another effective way to control the
work function is via chemical doping, e.g. Au-doping through
the electrochemical deposition.261 The resulting Au-doped
graphene films exhibit a change in work function of up to
B0.5 eV (i.e. from 4.66 eV to 3.96 eV), which leads to more
than 40 times increase in PCE as compared to the un-doped
graphene films.
7.4.2 Polymer based solar cells. In polymer-based hetero-
junction solar cells, graphene-based materials have been compo-
sited and incorporated to function as the electrodes,19,33,217,262264
electron transporters/acceptors,225,265267 and hole transporters.218
a. Transparent electrodes. Graphene thin films with easy
accessibility, good flexibility and transparency have shown
promises to replace indium tin oxide (ITO) as the transparentelectrode for solar cells and other electronic devices. In a
typical fabrication process, GO solutions are spin-coated on
quartz substrate and reduced chemically followed by high
temperature annealing. Additional layers in the device con-
figuration are deposited on top of the rGO film in sequence to
give the organic (e.g. quartz/rGO/P3HT/PCBM/PEDOT:PSS/
LiF/Al)217 or organic/inorganic hybrid (e.g. quartz/rGO/ZnO/
P3HT/PEDOT:PSS/Au)19 photovoltaic devices (Fig. 9A).
High temperature annealing not only improves the conductivity
of the rGO film,217,264 but also increases the work function of
rGO, resulting in a better match between the Fermi-level of the
rGO layer and the conduction band of the adjacent
Fig. 9 (A) Schematic illustration of the rGO thin film used as the
transparent electrode in a ZnO/P3HT hybrid solar cell. Reproduced
with permission from ref. 19. Copyright 2010, John Wiley & Sons, Inc.
(B) Schematic illustration of the P3HT/graphene hybrid film used as
the electron transport/acceptor layer in a polymer solar cell. Repro-
duced with permission from ref. 267. Copyright 2009, John Wiley &
Sons, Inc. (C) Left: schematic illustration of GO used as the hole
transport layer in a polymer solar cell. Right: Currentvoltage
characteristics of photovoltaic devices with no hole transport layer
(curve labeled as ITO), with a 30 nm PEDOT:PSS layer, and a 2 nm
thick GO film. Reproduced with permission from ref. 218. Copyright
2010, American Chemical Society.
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semiconducting layer for efficient charge injection.19 Besides
the work function, another key concern for the rGO electrode
is the transmittance. It is found that there exists a critical value
for the transmittance of the rGO electrode (e.g. 65%
in a device with a configuration of rGO/PEDOT:PSS/
P3HT:PCBM/TiO2/Al),33 above which the performance of
the device mainly depends on the charge transport efficiency
through the rGO electrodes, and below which the performance
is dominated by the light transmission efficiency. In addition
to rGO films, CVD-grown graphene films with better electrical
conductivity have also been used as the transparent electrode.
The performance of the CVD-grown graphene anode can
be improved by modulating its work function through
chemical means such as ozone treatment or non-covalent
functionalization.262 In the former approach, although ozone
treatment can generate the OH and CQO groups on the
graphene surface, thus improving the open circuit voltage
(Voc) and PCE, the electronic conductivity is compromised
due to the disrupted sp2 n